Phase change memory device having a diode that has an enlarged PN interfacial junction and method for manufacturing the same

1. A phase change memory device comprising:

a semiconductor substrate;

an insulation layer placed on the semiconductor substrate and having a contact hole in which the contact hole has a CD (critical dimension) that is greater at a middle portion than at respective upper and lower portions of the contact hole;

a diode formed within the contact hole; and

a phase change memory cell formed on the diode wherein the diode comprises a vertical PN diode which includes a P region stacked on top of an N region and wherein a junction surface between the N region and the P region is located in the middle of the contact hole.

2. The phase change memory device according to claim 1 , further comprising a conductivity type impurity region formed in a surface of the semiconductor substrate and electrically contacts to the diode.

3. The phase change memory device according to claim 1 , wherein the insulation layer has a triple-layered stacked structure comprising a first insulation layer, a second insulation layer, and a third insulation layer.

4. The phase change memory device according to claim 3 , wherein the second insulation layer comprises a layer which exhibits an isotropic etch rate higher than that of the first and third insulation layers of the insulation layer.

5. The phase change memory device according to claim 3 , wherein the first and third insulation layers comprise PE-TEOS (plasma enhanced tetra ethyl ortho silicate) layers, and the second insulation layer comprises a PSG (phosphosilicate glass) layer.

6. The phase change memory device according to claim 1 , wherein the diode comprises a silicon epi-layer.

7. A method for manufacturing a phase change memory device, comprising the steps of:

forming an insulation layer on a semiconductor substrate, the insulation layer defining a contact hole which has a CD (critical dimension) that is greater at a middle portion than at either an upper or a lower portion and a lower portion of the contact hole;

forming a diode within the contact hole wherein the step of forming the diode comprises the steps of: forming a stack structure of an N region and a P region in the contact hole wherein a junction surface between the N region and the P region is located in the middle of the contact hole; and

forming a phase change memory cell on the diode.

8. The method according to claim 7 , further comprises the step of forming a conductivity type impurity region in a surface of the semiconductor substrate before the step of forming the insulation layer.

9. The method according to claim 7 , wherein the step of forming the insulation layer having the contact hole comprises the steps of:

forming the insulation layer on the semiconductor substrate, the insulation layer having a triple-layered structure composed of a first, second, and third insulation layers in which the second insulation layer exhibits an isotropic etch rate greater than that of the first and third insulation layers;

anisotropically etching the insulation layer to expose a portion of the semiconductor substrate; and

isotropically etching the insulation layer which has been anisotropically etched so that the second insulation layer is etched more than the first and third insulation layers.

10. The method according to claim 9 , wherein the first insulation layer and the third insulation layer comprise PE-TEOS (plasma enhanced tetra ethyl ortho silicate) layers, and the second insulation layer comprises a PSG (phosphosilicate glass) layer.

11. The method according to claim 7 , wherein the diode is a vertical PN diode.

12. The method according to claim 7 , wherein the step of forming the diode comprises the steps of: growing a silicon epi-layer within the contact hole; and

forming the stack structure by conducting multiple ion implantation processes onto the silicon epi-layer within the contact hole.